In the limestone-based flue gas desulfurization (FGD) scrubber typical of most coal-fired electric power utilities, the limestone is used in the scrubbing process to remove combustion by-products during steam generation. Sulfur dioxide is the primary target of this scrubbing process and is produced when coal with high sulfur content is burned in the boilers. The selenium that is derived from the particular coal used as fuel to the boilers will be found in fly ash, flue gas emissions, scrubber solids, and in the scrubber liquids. Additionally other heavy metals will accumulate in the scrubber water. The selenium concentrations found in FGD scrubber liquids will vary from 0.5 to several parts per million (ppm).
The present invention is a process and system that will remove selenium from water that contains high concentrations of dissolved solids, and is an improvement upon the method disclosed in U.S. Pat. No. 4,806,264 (Feb. 21, 1989). The method described in U.S. Pat. No. 4,806,264 uses the ferrous ion to reduce the oxy-anions of selenium (i.e., selenate [SeO.sub.4.sup.2- ] and selenite [SeO.sub.3.sup.2- ]) to elemental selenium (Se.degree.) and co-precipitates the selenium with the resultant ferrous and ferric hydroxide sludges produced during the reaction. However, the application of the method described in U.S. Pat. No. 4,806,264 has not been found satisfactory by the present inventors for the removal of the selenium present in FGD scrubber purge water. Typical FGD scrubber liquids contain high concentrations of calcium, magnesium, sulfate, and total dissolved solids (TDS). With certain coals, the TDS will contain high concentrations of chlorides, on the order of 15,000 to 20,000 ppm, as chloride. In addition, this purge water may contain reduced forms of sulfur such as sulfite (SO.sub.3.sup.2-), thiosulfate (S.sub.2 O.sub.3.sup.2-), bisulfite (HSO.sub.3.sup.-), metabisulfite (S.sub.2 O.sub.5.sup.2-), dithionate (S.sub.2 O.sub.6.sup.2-), trithionate (S.sub.3 O.sub.6.sup.2-), and tetrathionate (S.sub.4 O.sub.6.sup.2-); organic acids such as adipic, glutaric, and succinic acids; and sulfur-nitrogen compounds such as hydroxylamine disulfonate (HADS), hydroxylamine sulfonate, and amine disulfonate. The sulfur-nitrogen compounds are formed through a reaction between the NO.sub.2 and SO.sub.2 absorbed from boiler flue gas.